Microorganisms have activity of conversion of substances based on excellent catalytic action. Thus, microorganisms have been extensively used in the fields of, for example, brewing, fermented food production, and wastewater or gas treatment. Recently, production of pharmaceutical products with the use of recombinant microorganisms, fermentative production of bioethanol, and synthesis of chemical products with the use of microorganisms have been implemented or researched, and techniques involving the use of microorganisms are industrially important. However, production of microbial cells that play key roles in reactions is costly in terms of, for example, media and energy. In addition, microbial reactions are often carried out in aqueous solutions, which necessitate the separation of microorganisms from reaction solutions after substances are produced. Microorganisms were mainly separated via centrifugation or filtration in the past.
In order to facilitate separation and continuously or repeatedly use valuable microbial cells, immobilization and autoagglutination of microbial cells were considered to be effective. Examples of conventional immobilization techniques include immobilization through entrapment in a gel such as alginic acid and surface immobilization to allow microorganisms to adsorb on the surfaces of porous carriers. However, the entrapment immobilization method is disadvantageous in that transportation of oxygen and substrates in gel is often limited in the rate, gel is brittle in mechanical strength and thus is likely to be destroyed by agitation or the like, and microbial cells leak from gel, for example. Also, a conventional technique of surface immobilization does not involve accumulation of target microorganisms on a surface, and the technique merely involves introduction of porous carriers or the like for use in the fields of wastewater processing or environmental cleanup where large quantities of many sorts of microorganisms are present, thereby allowing easily-adhering microorganisms to be carried on a surface as a biofilm. That is, there is no technique that allows microorganisms of interest to adhere to a solid surface as one likes. Regarding agglutination, a method involving the use of coagulating agents, such as high-molecular-weight polymers, is extensively used in wastewater processing. In addition, a method involving screening of agglutinating microorganisms for use thereof has been employed. However, there is no technique that enables spontaneous agglutination of non-agglutinating microbial cells, and particularly bacteria.